1. Field of the Invention
The present invention relates to optical pickups, more particularly to an optical pickup using a fiber type light-wavelength converting element, and also relates to an optical information recording carrier and a recording and reproducing apparatus thereof.
2. Description of Background Information
An example of optical pickups of that type is known in which the wavelength of a laser light beam generated from a laser light source is converted to a half using a light wavelength converter so that writing and reading of information for an optical disk can be performed at a higher density (reference is directed to Japanese patent application laid-open No. P61-50122).
A fiber type SHG (Second Harmonics Generator) using a secondary non-linear optical effect is an example of the light wavelength converter, and it adopts the phase matching of Cerenkov radiation. According to this system, second harmonic wave (hereinafter referred to as "SH wave") whose phase matching is performed nearly automatically can be generated.
In the fiber type SHG using the Cerenkov type of phase matching, the radiated SH has an intensity distribution of a ring shape and its equiphase surface is of a conical shape.
FIG. 1 shows an example of an optical pickup using the fiber type SHG in the prior art. In FIG. 1, a primary ray generated from a light source 1 such as a semiconductor laser is converged by a condenser lens 2 and incident to a fiber type SHG 3. The SH ray subjected to a wavelength conversion by the fiber type SHG 3 into half the wavelength of the primary ray is issued on the same optical axis and is incident an axicon lens 4. In the axicon lens 4, the SH ray is converted into parallel rays. The SH ray converted into the parallel rays passes through a primary-ray cut filter 5, while the primary ray is interrupted by the filter 5. Since the polarization plane of the SH wave is of the P polarization, the SH wave passes through a beam splitter 6 and the polarization plane is converted from linear polarization into circular polarization at a .lambda./4 plate 7. Then the SH ray is converged by means of an objective lens 8 onto an information recording surface, for example, of an optical disk 9, i.e., an information recording carrier.
The reflected ray from the optical disk 9 passes through the objective lens 8 and its polarization plane is changed from the circular polarization into the linear polarization in the .lambda./4 plate 7. Then, the reflected ray is incident on the beam splitter 6. Since the incident ray of the beam splitter 6 passes through the .lambda./4 plate 7 twice in the outward and return paths, the polarization plane becomes S polarization, so that the incident ray is reflected by the beam splitter 6 and passes through a condenser lens 10 and a cylindrical lens 11 and is incident on a light receiving surface of a so-called quadrants type light detector 12. The total sum of four outputs of the quadrants type light detector 12 is derived as a read RD signal, and further, by combination of the four outputs based on a known error producing method, focus error and tracking error signals are derived.
In the optical pickup using the fiber type SHG 3 in the prior art, since the primary ray is removed by the filter 5 and only the SH ray is used, such optical pickup can be used for playing the disk of high density but cannot be used for playing the optical disk on which information is recorded using the primary ray.
On the other hand, in an optical disk such as an optical information recording carrier (including a photomagnetic disk and a phase changing type disk), the diameter of the spot Of the information reading ray must be decreased in order to raise the density of the recorded information. Consequently, in the prior art, the wavelength of the information reading ray is decreased as described above, or the numerical aperture NA of the objective lens for converging the information reading ray onto the information recording surface of the optical disk is enlarged, so that the diameter of the spot of the information reading ray is reduced and information is recorded at a high density. However, the wavelength of short-wavelength light sources of a reduced diameter is about 400 nm and the numerical aperture NA of the objective lens is about 0.6 in consideration of the practical use of the system. Consequently, limitation exist when an attempt is made significantly improve the record density by decreasing the spot diameter of the information reading ray.